1. Field of the Invention
The invention relates generally to memory structures, and more particularly to the design, fabrication, and use of memory structures that comprises embedded flash memory.
2. Background of the Invention
Many conventional memory devices use flash memory cells. Many conventional flash memory cells use floating gate technology to store one or more bits of information in the floating gate when a program voltage is applied. The operation of floating gate flash memory devices is well known and will not be discussed herein for the sake of brevity. More recently, floating gate technology has been displaced by the use of other technologies that can be scaled to meet increasing memory density demands. For example, SONOS technology has become more prevalent in many applications. In a SONOS cell, the cell comprises a silicon layer (S), an oxide layer (O), a nitride layer (N), another oxide layer (O), and another silicon layer (S). Appropriate programming voltages applied to the SONOS stack causes a bit of data, or a charge, to be stored in the nitride layer. By applying the appropriate read voltages to a SONOS cell, it can be determined whether the cell has been programmed.
While there have been advancements in conventional memory cell design, such as the development of SONOS flash memory, new applications are constantly driving new memory requirements that cannot necessarily be met by the use of conventional memory structures.
As such, demands are likely to continue, and even increase, in the future, it is important to develop new techniques for memory structure design and fabrication. One such technique comprises stacked thin-film memory cells. Thin-film deposition is any technique for depositing a thin film of material into a substrate or onto previously deposited layers. “thin” is a relative term, but most such deposition techniques allow layer thickness to be controlled within a few hundred nanometers, and some allow one layer of atoms to be deposited at a time. Thus, thin-film structures can be used to reduce the overall size and allow increased density, e.g., by stacking thin-film structures. Unfortunately, the thin-film devices are not necessarily as reliable as devices constructed using more conventional deposition techniques. Thus, the applicability of thin-film structures in memory applications is limited due to their inherent unreliability.